The Moon



The moon and Earth share a few interesting characteristics:

--they are about the same distance from the sun


--they are mostly comprised of material similar, if not identical, to the Earth's Mantle.


--Men have walked on their surfaces


Lunar Orbit:

--The plane of the Moon's orbit is tilted five degrees relative to the Earth's orbital plane.


--The line joining the Earth and the Sun is called the line of nodes. That's where the two planes intersect.


--Eclipses occur when the moon is on that line.


--Note that the orbit of the moon is an Ellipse, just like the planets.


The Earth & Moon Orbital System:

Both the Earth and moon orbit the center of mass of the Earth Moon system. This point is at one focus of the Moon's orbital ellipse. It is also at one focus the orbital ellipse traced out by the center of the Earth (note that this point is actually within the Earth)


--The semi major axis of the Moon's orbital ellipse is 384,000 km, but the orbit is eccentric with the perigee distance of 363,000 km, and the apogee distance of 405,000 km.


--This difference of about 10 percent changes the moons angular diameter in the sky from about .53 degrees to about ..58 degrees.

Can this cause the Full Moon to appear larger at times?


--This angular difference results in annular eclipse when the moon is at apogee, and total eclipses when the moon is at perigee.


--Because of the Elliptical orbit, the moon moves faster near Perigee, and slower near apogee. Thus, some phases last longer then others.

Only One Face shows?

The Moon rotates around the Earth once every 27.3 days (this is the true period of the moon or the sidereal month). It also spins on its axis once every 27.3 days. How can this be? The reason is the same as the reason why we have tides on the Earth. The moon is tidally "locked" with one side always facing the Earth.


--Since the Moon is oblate (like a football) if it tried to turn, the greater force on the near side would line back up again. This means, in principle, that one side of the moon is always facing the Earth.


-- In reality, the moon appears to rock back and forth slightly because of its varying orbital speed.


--This libration causes 59% of the Moons surface to be visible, rather then 50 %.


--It wasn't until we sent unmanned probes to the moon that we saw the back side. Surprisingly, the back side is somewhat different then the front (No Maria at all!)

Lunar Gravity:

If you've seen astronauts bouncing around on the Lunar Surface, you known that there is a smaller force of gravity acting on them there then there is on Earth.

The easiest way to express this difference is the acceleration due to gravity "g".


--On Earth, g is 9.8 m/s2 . On the moon g is about 3.3 m/s2 or about 1/6 what it is on Earth. This is because the mass of the moon is much less then the mass of the Earth (1/80). This is partially compensated for by the radius of the moon being somewhat smaller then the Earth's (1/3). So is you weigh 180 lb. on Earth, you will weigh 1/6 of 180 or 30 lb. on the Moon.


- On the Last Apollo mission to the moon, Alan Shepard brought golf ball and (allegedly) hit it over the (smaller) horizon.

Atmosphere and Temperature:

You've probably noticed that astronauts had to wear bulky, if not heavy. space suits. This was to supply them with oxygen, and, to keep them pressurized.


--Because of the lower gravity on the moon, and daytime temperatures as high as 125C (water boils at 100C), whatever gasses were once near the moons surface have long ago escaped into space (how does this compare with the escape of helium and hydrogen from the Earth's atmosphere ?)


--Without any atmosphere, that means that the air pressure is near zero. In other words, without suits, the astronauts would have 14.7 lb./in2 pushing out, and nothing pushing in, so they would explode (actually, their fluids would leak explosively into space) !



If we divide the mass of the moon by its volume, we get a density of 3.3 gm/cm3 .


--This is the same as the density of the Mantle on the Earth, but less then the overall density of the Earth

(5.5 gm/cm3 ).


--This means that there are few, if any, metals in the core of the Moon. (Why?)

Lunar Geology:

--Apollo Missions 11 - 17, from 1969 - 1972 brought back 382 kg (842 lbs). They differed significantly depending on whether they came from Maria (lowlands), or Terra (highlands). However, all had the following traits in common:

Rock Samples from Maria are primarily Dense Basalt, meaning that they were produced in lava flows (as in the Hawaiian islands), they are richer in heavy elements such as Iron, manganese, and Titanium, and that they are dark in color.

Rock samples from the Terra are generally Anorthosite, meaning rock that is rich in calcium, aluminium, and Oxygen (i.e. silicates of lighter metals).





-- In both areas, there are craters, but there are more numerous in the highlands.


--There are 14 Maria on the near side, and none at all on the far side.


--This discrepancy provides some evidence about how the moon was formed.


-- The Terra and the Maria have significantly different ages. This can be easily seen by the relative lack of large craters. The Terra regions have been dated from about 4 to 4.4 billion years old, while the Maria date from 3.1 to 3.9 billion years (the "or" statment is due to uncertainties in the dating process).


-- Before Apollo, most people thought that the Maria were newer "seas" of lava that had upwelled through cracks in the Lunar crust and solidified, covering most of the craters. They were correct about that, but they didn't realize that the two regions were so close in age, implying that most of the heavy cratering occurred before 3.9 billion BC , and abruptly ended about that time. The same would be true for the Earth as well.


--Radioactive dating tells you how long ago the rock solidified, so it works very well for dating regions such as Maria and Terra.

Cratering and Dust:

The Lunar Surface tells the story of the subsequent four billion years, mostly in the depth of the dust, called regolith.


--This dust is the result of meteorite impacts of all sizes that pulverize the surface and eject material in all directions. To give an idea of the magnitude of this process, the depth of the regolith in the lunar highlands can reach 100 meters.

--The solar wind (high energy particles from the sun) also contributes to the regolith.

--Its depth is somewhat less in the Maria, as thin as 10 meters.

--Even though the period of intense bombardment ended almost four billion years ago, meteorites of all sizes continue to strike the Earth and the moon.

--While smaller meteorties typically burn up in the Earth's atmosphere, they strike the moon unimpeded.


Impact frequency:

Meter sized craters are formed about once a month, while cm sized craters are formed every few minutes. As far as we know, the smaller the crater, the more often they occur. The really big craters (say 10 km in diameter) are formed roughly every ten million years. The same holds true for the Earth.



While water, and wind are the primary forces of Erosion on the Earth, this meteoric rain is the only real erosive agent on the moon. If you could describe an erosion rate by how long it would take a particular feature, say a footprint left in the dust, to disappear, you could saw that the moons erosion rate is 10,000 times less then the Earth's. Obviously, then the Astronaught footprints will be around for a long time.

How the Moon Formed:

The hybrid capture theory is the current favorite. It postulates that a Mars-sized object struck the Earth and left its core behind, sending mantle material out into space. Computer studies indicate that the ejecta would form into a spherical object (our Moon) in a stable orbit.


--Such collisional events were apparently quite common during the early epochs of our solar system.

Water on the Moon?

Radar images released by the Department of Defense in 1997 (and their Clemintine spacecraft) suggested that water ice may be present in at least one large crater very close to the Lunar South Pole. Because of the high rim of this crater, no sunlight can penetrate to the crater floor and melt the ice. The subsequent mission, Lunar Observer confirmed water ice at the south Pole, and found significant deposits of water near the North Pole as well. Apparent the frozen water is mixed in the regolith, and would require "mining," to recover. Still, this is a much more appealing prospect than "trucking" the water up from Earth.

--It has long been suspected that water exists on the Moon as ice bearing meteorites must have impacted frequently over the last four billion years.

--For the time being, the crater rim near the South Pole is the most valuable piece of real estate on the Moon. "Water Ice and Midnight Sun--what more could you ask for in a Lunar estate?"